The present invention relates to a lithium secondary battery including spinel type lithium manganese oxide as a positive electrode active material and gelled polymeric electrolyte as an ion conducting medium, and more particularly, it relates to improvement of a positive electrode for the purpose of providing a lithium secondary battery using a gelled polymeric electrolyte with large initial discharge capacity and good charge-discharge cycle performance.
As an ion conducting medium (electrolyte) of a lithium secondary battery, a liquid electrolyte (electrolytic solution) has been conventionally used because of its good ionic conductivity although it has problems of leakage and elution of an electrode material.
When a liquid electrolyte is used as an ion conducting medium in a lithium secondary battery using lithium manganese oxide as a positive electrode active material, however, manganese included in the lithium manganese oxide is gradually eluted into the liquid electrolyte, resulting in causing a problem that the discharge capacity is degraded in a small number of charge-discharge cycles.
When a solid electrolyte (such as a film and a foil) is used instead of a liquid electrolyte, the degradation of the discharge capacity due to the elution of manganese into the electrolyte can be avoided. The ionic conductivity of the solid electrolyte is, however, generally lower than that of the liquid electrolyte, and a contact area between the electrolyte and the electrode is so small that the electric resistance (interface resistance) on the interface between the electrolyte and the electrode is large. Therefore, the discharge capacity, at high rate discharge in particular, is degraded.
Accordingly, as an ion conducting medium for improving the disadvantages of and making the best use of the advantages of the liquid electrolyte and the solid electrolyte, a gelled electrolyte, particularly a gelled polymeric electrolyte that can be easily formed into a thin film and is inexpensive, has been recently proposed. A gelled polymeric electrolyte is a gelled substance obtained by impregnating a liquid electrolyte including a solute (electrolytic salt) and a solvent into a matrix of a polymer (resin). Since a gelled polymeric electrolyte includes a liquid electrolyte, it has higher ionic conductivity than a solid electrolyte, and since the liquid electrolyte is fixed through gelation within the matrix of the gelled polymeric electrolyte, manganese is minimally eluted into the liquid electrolyte.
When a gelled polymeric electrolyte is used, however, a contact area between the electrode and the electrolyte is smaller than in using a liquid electrolyte. Therefore, the electric resistance (interface resistance) on the interface between the electrode and the electrolyte is large as in using a solid electrolyte. As a result, the discharge capacity, at high rate discharge in particular, is degraded.
Accordingly, an object of the invention is providing a lithium secondary battery using a gelled polymeric electrolyte with large initial discharge capacity and good charge-discharge cycle performance.
The lithium secondary battery using a gelled polymeric electrolyte of this invention (present battery) comprises a positive electrode including a gelled polymeric electrolyte (A) and using spinel type lithium manganese oxide as an active material; a negative electrode; and a gelled polymeric electrolyte (B) in the shape of a film or sheet also serving as a separator, and the gelled polymeric electrolyte (A) and the gelled polymeric electrolyte (B) are made from a polymer of poly(alkylene oxide) series impregnated with a liquid electrolyte.
Both the gelled polymeric electrolyte (A) included in the positive electrode and the gelled polymeric electrolyte (B) in the shape of a film or sheet also serving as a separator are the polymers of poly(alkylene oxide) series impregnated with a liquid electrolyte.
Examples of the polymer of poly(alkylene oxide) series are poly(ethylene oxide), poly(propylene oxide), a block copolymer of poly(ethylene oxide) and polystyrene, a block copolymer of poly(ethylene oxide) and polypropylene oxide, polyetherimide, polyethersulfone, polysiloxane and polysulfone. From the viewpoint of the charge-discharge cycle performance, the block copolymer of poly(ethylene oxide) and polystyrene is particularly preferred. In particular, the polymer of poly(alkylene oxide) series used for the gelled polymeric electrolyte (B) preferably has high mechanical strength and a large molecular weight. When, for example, poly(ethylene oxide) is used, the number average molecular weight Mn is preferably approximately two million through eight million.
Examples of the electrolyte used for impregnating the polymer of poly(alkylene oxide) series are LiClO4, LiCF3SO3, LiPF6, LiBF4, LiSbF6, LiAsF6, LiN(CmF2m+1SO2)(CnF2n+1SO2) (wherein m and n independently indicate an integer ranging between 1 and 5), and LiC(CpF2p+1SO2)(CqF2q+1SO2)(CrF2r+1SO2) (wherein p, q and r independently indicate an integer ranging between 1 and 5). Examples of the solvent are ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, diethyl carbonate, methylethyl carbonate, xcex3-butyrolactone, sulfolane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-ethoxymethoxyethane, tetrahydrofuran, 2-methyl-1,3-dioxolane, 4-methyl-1,3-dioxolane, dimethyl ether, diethyl ether, ethyl acetate and methyl propionate. The liquid electrolyte preferably includes LiN(CmF2m+1SO2)(CnF2n+1SO2) (wherein m and n independently indicate an integer ranging between 1 and 5) and/or LiC(CpF2p+1SO2)(CqF2q+1SO2)(CrF2r+1SO2) (wherein p, q and r independently indicate an integer ranging between 1 and 5) in a concentration of 0.1 through 2.0 mol/liter because the elution of manganese can be thus effectively suppressed during charge-discharge cycles. When another solute is used together with LiN(CmF2m+1SO2)(CnF2n+1SO2) (wherein m and n independently indicate an integer ranging between 1 and 5) and/or LiC(CpF2p+1SO2)(CqF2q+1SO2)(CrF2r+1SO2) (wherein p, q and r independently indicate an integer ranging between 1 and 5), the concentration of the solutes in the liquid electrolyte is preferably lower than 2.0 mol/liter.
In the case where the gelled polymeric electrolyte (A) and the gelled polymeric electrolyte (B) are made from the same material, a film of a polymer of poly(alkylene oxide) series is formed on a positive electrode by a casting method or the like, and part of the cast polymer of poly(alkylene oxide) series is allowed to be included in the positive electrode at the same time. Subsequently, the polymer of poly(alkylene oxide) series is impregnated with the same liquid electrolyte. In this manner, the film-like gelled polymeric electrolyte (B) also serving as the separator and the positive electrode including the gelled polymeric electrolyte (A) are preferably integrally fabricated because the fabrication can be thus eased and the contact resistance between the gelled polymeric electrolyte also serving as the separator and the positive electrode can be lowered.
The spinel type lithium manganese oxide used as the active material of the positive electrode is lithium manganese oxide having a spinel structure belonging to the cubic system. A specific example of the spinel type lithium manganese oxide is LiMxMn2xe2x88x92xO4 (wherein M is at least one element selected from the group consisting of Ni, Al, Mg, Fe and Co; and 0xe2x89xa6xxe2x89xa60.5).
Examples of the material for the negative electrode are a substance capable of electrochemically occluding and discharging lithium ions and metallic lithium. Examples of the substance capable of electrochemically occluding and discharging lithium ions are a carbon material such as graphite (natural graphite and artificial graphite), coke and an organic baked substance; lithium alloy such as lithiumxe2x80x94aluminum alloy, lithium- magnesium alloy, lithiumxe2x80x94indium alloy, lithiumxe2x80x94tin alloy, lithiumxe2x80x94thallium alloy, lithiumxe2x80x94lead alloy and lithiumxe2x80x94bismuth alloy; and a metal oxide or metal sulfide including one of or two or more of tin, titanium, iron, molybdenum, niobium, vanadium and zinc.
Since the present battery uses a positive electrode including a specific gelled polymeric electrolyte (A), the contact area between the positive electrode active material and the gelled polymeric electrolyte is large. Accordingly, the present battery can attain large initial discharge capacity (at high rate discharge in particular). Also, since the present battery uses a specific gelled polymeric electrolyte (B) as the electrolyte, manganese included in spinel type lithium manganese oxide is minimally eluted, which can reduce the degradation of the discharge capacity derived from elution of manganese during charge-discharge cycles. Accordingly, the present battery can exhibit good charge-discharge cycle performance.